Nitride semiconductors (Group III nitride semiconductors) containing, as a main component, gallium nitride (GaN), aluminum nitride (AlN) or indium nitride (InN), or a mixed crystal of these compounds, are wide band gap semiconductors, and are large in dielectric breakdown field. Moreover, the semiconductors are larger in electron saturated drift velocity than silicon-based semiconductors or gallium arsenic (GaAs)-based compound semiconductors. Thus, the nitride semiconductors can gain a high electron mobility and be further heightened in withstand voltage. Furthermore, between a hetero-interface between aluminum gallium nitride (AlGaN) having, as a main surface thereof, a surface having a (0001) plane direction, and gallium nitride (GaN) or the like, electric charges are generated by spontaneous polarization or piezoelectric polarization. The sheet carrier concentration in the hetero-interface becomes 1×1013 cm−2 or more by these polarization effects even when this semiconductor is not particularly doped. It is therefore possible to make use of a two-dimensional electron gas (2DEG) in the hetero-interface to realize a hetero-junction field effect transistor (HFET) which is high in current density.
At present, as a substrate on which crystals of such a nitride semiconductor are to be grown, there is used not a substrate made of a nitride semiconductor same as the crystal growing material, but a sapphire substrate, a silicon carbide substrate, a silicon substrate, or any other substrate made of a material different in kind from the crystal growing material and having a large lattice mismatch with nitride semiconductors. This is because even when a substrate made of a nitride semiconductor is produced, it is necessary to form, by a vapor growth method, the nitride semiconductor on a substrate made of a material different in kind from the nitride semiconductor to be grown. In other words, in order to grow a nitride semiconductor on a substrate of a nitride semiconductor same in kind to the crystal growing material to produce a substrate made of the nitride semiconductor, costs are high in the present circumstances and further a substrate having a large diameter is not obtained. By contrast, according to silicon substrates, substrates having a large diameter can be mass-produced, so that the silicon substrates are superior also from the viewpoint of costs.
However, when a nitride semiconductor is formed on a silicon substrate, the following drawbacks are caused.
First, the nitride semiconductor is larger in thermal expansion coefficient than silicon, and a difference therebetween is also large. Moreover, crystal growth of the nitride semiconductor is generally conducted at a high temperature of about 1000° C.; thus, when a film of the nitride semiconductor is formed on the silicon substrate at a high temperature and subsequently the temperature of the substrate is lowered to room temperature, tensile stress is easily generated in the nitride semiconductor by a difference in thermal expansion coefficient between the nitride semiconductor and the silicon substrate. Accordingly, there arises a problem that highly-dense defects, or cracks are generated in the nitride semiconductor formed on the silicon substrate.
Second, when a nitride semiconductor containing gallium (Ga) is formed, a raw material therefor is easily combined with silicon to produce a compound. Accordingly, there also arises a problem that when a nitride semiconductor containing gallium (Ga) is formed on a silicon substrate, the nitride semiconductor hardly grows flatly on the silicon substrate.
Third, a nitride semiconductor grown on a substrate different in kind from the nitride semiconductor which is a crystal growing material, such as a silicon substrate, has a problem of being affected by lattice mismatch with the substrate, so as to become very high in dislocation density.
In order to restrain such problems of silicon substrates, various methods have been investigated as a method for forming a film of a nitride semiconductor on a silicon substrate. For example, investigations have been made about the formation of an aluminum oxide layer on a silicon substrate by sputtering, followed by the formation of an aluminum nitride layer on the aluminum oxide layer (see, for example, PTL 1).
Moreover, investigations have been made about the formation of a nitride semiconductor by nitriding silicon on a silicon substrate to produce silicon nitride, and then forming aluminum nitride on the silicon nitride (see, for example, PTL 2).
Furthermore, investigations have been made about an improvement of the crystallinity of aluminum nitride by forming aluminum nitride on a silicon substrate having a main surface having an inclination angle of 0.1° or less to a (111) plane of the silicon substrate (see, for example, PTL 3).